1. Field of the Invention
The present invention relates to a method and apparatus for standardized image generation which detects reradiated light of mutually different wavelength bandwidths, generated from a living tissue by irradiation thereof by light, as images; adds a desired offset to at least one of the images of mutually different bandwidths; and generates a standardized image based on the ratio between the mutually different images, at least one of which the offset has been added to.
2. Description of the Related Art
When a living tissue is irradiated by excitation light within an excitation light wavelength range of pigment existing within a living organism, normal tissue and diseased tissue emit fluorescent light of different intensities. There has been proposed a fluorescence detection apparatus that takes advantage of this phenomenon by irradiating a living tissue with excitation light of a predetermined wavelength range, and recognizing the location and region of infiltration of diseased tissue by detecting the fluorescent light emitted by the pigment existing therein.
Generally, when irradiated with excitation light, normal tissue emits strong fluorescence as indicated by the solid line in the graph of FIG. 10, whereas fluorescence weaker than that of normal tissue is emitted from diseased tissue, as indicated by the broken line in the graph of FIG. 10. Therefore, by measuring the intensity of fluorescence, a judgment can be made as to whether living tissue is normal or in a diseased state.
Further, there has been proposed a method in which fluorescence caused by excitation light is imaged by an imaging element, and a judgment is made as to whether living tissue is normal or in a diseased state by displaying a fluorescence image corresponding to the intensity of the fluorescence. With regard to this technology, as there are concavities and convexities in living tissue, the intensity of excitation light irradiated on the living tissue is not uniform. Consequently, the intensity of the fluorescence emitted from the living tissue decreases in proportion to the square of the distance between a light source and the living tissue. In addition, the intensity of the fluorescence emitted from the living tissue decreases in proportion to the square of the distance between a fluorescence detection means and the living tissue as well. Therefore, there are cases in which fluorescence of a stronger intensity is received from diseased tissue closer to the light source or the fluorescence detection means than from normal tissue further therefrom. Consequently, it is not possible to accurately discriminate the tissue state of living tissue based solely on information regarding the intensity of fluorescence caused by excitation light. In order to reduce such inaccuracies, there has been proposed a method in which fluorescence images are obtained based on fluorescence intensities of two mutually different wavelength bandwidths (a narrow bandwidth in the vicinity of 480 nm, and a wide bandwidth ranging from the vicinity of 430 nm to the vicinity of 730 nm), the ratio between the two fluorescence images is derived by division, then a standardized fluorescence image is generated based on the divided value. That is, a method of standardized fluorescence image generation based on the difference of the shape of the fluorescence spectra which reflects the tissue state of an organism has been proposed. Further, a method in which color information is assigned to the divided value of the fluorescence images of different wavelength bandwidths, and indicating the diseased state of living tissue by differences in color shown in a color image has been proposed. Still further, a method in which a near infrared light, which is uniformly absorbed by various living tissues, is irradiated on living tissue as a reference light, a reference image based on the intensity of the light reflected by the living tissue upon irradiation thereof with the reference light is detected, brightness information is assigned to the reference image, and the brightness image obtained thereby is synthesized with the aforementioned color image, thereby showing an image having a three dimensional feel which reflects the contour of the tissue as well, has been proposed.
In addition, in the case that a standardized fluorescence image is generated based on the divided value of fluorescence images of different wavelength bandwidths, the fluorescence intensity from the living tissue used for the standardization calculation is minute. Therefore, the standardized fluorescence image based on this fluorescence intensity has an extremely low S/N ratio. In order to ameliorate this problem, the adding of an offset to at least one of the fluorescence images of different wavelength bandwidths when the aforementioned divided value is calculated, thereby improving the S/N ratio of the standardized fluorescence image, has been proposed.
When deriving a divided value by adding an offset as described above, a favorable S/N ratio can be obtained by making the value of the offset large. However, the fluorescence intensity changes greatly depending on the distance between the detection means and the living tissue. Therefore, if the offset is made too large, cases arise in which living tissue far from the detection means and living tissue close to the detection means have greatly different divided values even when their diseased state is the same. Consequently, the discrimination between diseased tissue and normal tissue becomes difficult. As an example, the relationship between the divided values and the aforementioned distances of a predetermined normal tissue and a predetermined diseased tissue is shown in the graph of FIG. 8. The divided values were obtained by dividing a narrow bandwidth fluorescence image by a wide bandwidth fluorescence image to which an offset was added (narrow bandwidth fluorescence image/(wide bandwidth fluorescence image+offset)). Values of 5, 10, 15, and 20 were utilized for the offset, and the divided values for the normal tissue are indicated by the outlined symbols, while the divided values for the diseased tissue are indicated by the solid symbols. As shown in the figure, the divided value changes according to the aforementioned distance. It can be seen that the larger the offset value, the difference between the divided values of the normal tissue and the diseased tissue decrease as the aforementioned distance increases, and discrimination therebetween becomes difficult. Conversely, if the offset value is small, changes in the divided value according to distance is less significant. However, a problem arises that an effect whereby the S/N ratio of the standardized fluorescence image is improved is not sufficiently obtained. Further, standardized fluorescence images of a predetermined living tissue based on the above described divided value are shown in FIGS. 9A and 9B. FIG. 9A shows a standardized fluorescence image wherein the distance between the detection means and the living tissue is large, and FIG. 9B shows a standardized fluorescence image wherein the aforementioned distance is small. Note that the offset value in these cases is 20, and in each image, the portions that are comparatively darker than their surroundings indicate diseased portions. As shown in the figure, although it is necessary that the large distance image (far image) and the small distance image (close image) display their normal tissue portions and their diseased tissue portions at the same brightnesses, respectively, the far image is darker than the close image as a whole. In addition, there is little contrast in the far image between the normal tissue portion and the diseased tissue portion, making discrimination therebetween difficult.